As is known, the speed of a helicopter is a function of main rotor collective pitch and longitudinal cyclic pitch. In helicopter autopilot systems, it is known to provide airspeed hold at cruise speeds (such as above about 60 knots) and to provide aircraft attitude hold at speeds below cruise speeds. In order to hold speed, a desired speed is achieved and then the hold system is synchronized to it, so that the desired speed is "remembered" and deviations in actual speed provide inputs to the helicopter control system so as to correct the speed until the error is zero. Similarly, desired attitude can be synchronized when attitude hold is employed. The system for responding to deviation from either the desired speed or the desired attitude generally includes both proportional and integral gains to provide fast, stable operation with zero steady state error.
Also known in the helicopter art is what is sometimes referred to as a "beeper", which derived its name from limited pulsing of hydraulic servo-mechanisms in response to the depression of a beeper valve, which causes a hydraulically-induced beep-like noise. In more modern, electrical control systems, the term "beeper" refers to spring returned switches, for both the forward and aft directions (in the case of longitudinal cyclic pitch controls), that permit the pilot to nudge an autopilot output for either an increase or decrease in pitch attitude. In most helicopter autopilot systems known to the art, the beeper system is effective only in the pitch attitude channel, and not in the airspeed hold channel. Thus, nudging of airspeed is accomplished by nudging pitch attitude in anticipation of an ultimate airspeed which will result from the attitude assumed by nudging with the beeper switch. But, for desired acceleration, the required pitch attitude is achieved only by ramping the autopilot system (usually integrated attitude error) way beyond the value of a desired airspeed reference. Thus, opposite beeping is always required not only to reduce acceleration to zero at a desired speed, but also to reduce the integrated error reference to near zero before removing airspeed synchronization.
The response of a helicopter to inputs to its control system results in inherent lags. For instance, there is a lag between the command to achieve a particular attitude and the stabilization of the aircraft at such attitude. In addition, once a pitch attitude is actually achieved by a helicopter, the speed may continue to increase (or decrease) until an equilibrium speed, for the given attitude, is achieved. Therefore, if the pilot is using a pitch attitude beeper to adjust his airspeed, he must anticipate the changes in airspeed which will result after the beeper switch is released and the new attitude is established. Therefore, prior art helicopter control systems require a heavy pilot workload to estimate desired speed changes achievable by use of attitude beeping, while followed by one or two iterations of corrections thereto until desired airspeed has been achieved. In some systems, the anticipation of final aircraft speed prior to reingagement of the airspeed hold system has been achieved by means of a delay in reingagement of airspeed hold, such as on the order of a half a minute or so. However, this improvement requires that the pilot at least wait during that time frame before providing any additional trimming which may be necessary; furthermore, during turbulent conditions, it is impossible to acquire a precise airspeed reference at the point of reingagement of the airspeed hold system.
Another problem with prior art systems is that when beeping of attitude is used for speed correction, resynchronizing of the memorized desired airspeed at the initiation of beeping causes a discontinuity in the input to the control system. The degree of disturbance which results is a function of the amount of airspeed error at the time the beeping is initiated.
In some systems, a beeper switch may be applicable to an airspeed hold system (rather than only to the attitude hold system) allowing the pilot to beep his airspeed reference while at cruise speeds. However, once beeping is initiated, the lag in the speed response of the aircraft to attitude changes renders it impossible for the pilot to know what the new airspeed reference point will be until after speed has been stabilized. Although judgments can be made, it requires a high pilot workload to anticipate the amount of beeping required to change a speed reference point, and several corrections once the speed does begin to stabilize at a new reference point. This disadvantage is even more significant when large changes in airspeed are commanded by means of the beeper.
Another characteristic of helicopter airspeed hold systems known to the prior art, which results from the lags inherent in the system, is that any airspeed hold system must operate at high gain in order to be able to fully correct for variations in airspeed so as to hold the speed relatively constant as desired. However, if the gain of the airspeed hold system is increased to the point where it will maintain relatively constant speed in calm air, the system then becomes too responsive to gusts and turbulence, resulting in an uncomfortable ride. Thus, a compromise must be reached between the capability of maintaining airspeed and undesirable, erratic response during gusts and turbulence, resulting in a somewhat uncomfortable ride, coupled with less than adequate airspeed retention.
Yet another problem with prior art systems is that the transfer between attitude hold and airspeed hold, as the airspeed transitions between cruise and subcruise speeds, causes the control system inputs to commensurately transfer from speed error to attitude error (or vice versa) resulting in discontinuity. Thus, if airspeed were decreasing while on airspeed hold, the transition from a speed above cruise to a sub-cruise speed will result in a disturbance in the aircraft attitude due to loss of the integrated airspeed error input, which could be significant.
Another problem with aircraft autopilot systems known to the prior art is that when the aircraft is on autopilot, operations of the pilot in performing a maneuver will normally require the pilot to reestablish any trim point which he desires for attitude or airspeed hold, and/or result in discontinuities in aircraft commands as a consequence of transitions between manual and automatic operation.